init: respect --repo-url changes

We respect this option when running the first `repo init`, but then
silently ignore it once the initial sync is done.  Make sure users
are able to change things on the fly.

Bug: https://crbug.com/gerrit/11045
Change-Id: I129ec5fec43a85067d555bb60c0d1ae02465f139
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258893
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>